1. Field of the Invention
The present invention relates to a method of producing a semiconductor member and a method of producing a solar cell. More particularly, the present invention relates to a method of producing a semiconductor member comprising a thin film crystal formed on an inexpensive substrate and a method of producing a solar cell with high performance using the semiconductor member.
2. Related Background Art
There has been extensively researched a solar cell as a power supply which is systematically coupled with a driving energy source of various equipment or a commercial power. The solar cell is so desired that a device can be formed on an inexpensive substrate to comply with a cost demand. On the other hand, silicon is typically employed as the semiconductor constituting the solar cell. In particular, from the viewpoint of the efficiency of converting light energy into an electric power, that is, from the viewpoint of the photoelectric conversion efficiency, single-crystal silicon is excellent. On the other hand, from the viewpoint of increase in area and reduction in costs, amorphous silicon is advantageous. Also, in recent years, for the purpose of obtaining costs as low as amorphous silicon and energy conversion efficiency as high as single-crystal, polycrystal silicon has come into use.
However, in single-crystal or polycrystal silicon, since a lump-shaped crystal is sliced into a plate-shaped substrate, it is difficult to make its thickness 0.3 mm or less. Thus, the material used for the substrate has not been effectively utilized because the substrate generally has a thickness much over a thickness necessary for absorbing incident light (20 .mu.m to 50 .mu.m). Also, in recent years, there has been proposed a method of forming a silicon sheet by using a spin method in which a liquid droplet of melted silicon is allowed to flow into a die. Even in this method, the thickness of the substrate is about 0.1 to 0.2 mm at the minimum, which is not sufficiently thin. In other words, there is room for further reduction of costs by further thinning silicon.
An attempt of achieving high energy conversion efficiency and low cost of a solar cell has been made in which a thin epitaxial layer grown on a single-crystal silicon substrate is separated (peeled) from the substrate (Milnes, A. G. and Feucht, D. L., "Peeled Film Technology Solar Cells", IEEE Photovoltaic Specialist Conference, p. 338, 1975). In this method, an intermediate layer of SiGe is interposed between single-crystal silicon for forming the substrate and the grown epitaxial layer, and a silicon layer is allowed to (hetero) epitaxially grow thereon. Therefore, the intermediate layer is selectively melted so that the growth layer is peeled off. However, in general, the hetero-epitaxially grown layer is liable to have defects induced on a growth interface because the layer is different in lattice constant from the substrate. Also, this method is not advantageous in terms of process costs because of the use of a material such as germanium which is remarkably more expensive than silicon.
Also, U.S. Pat. No. 4,816,420 discloses that a thin crystal solar cell is obtained by making selective epitaxial growth on a crystal substrate through a mask material, forming sheet-like crystal by using a method of allowing crystal to laterally grow, and thereafter separating the crystal from the substrate. However, in this method, since the sheet-like crystal is mechanically peeled off using cleavage, the sheet-like crystal is liable to be damaged during a peel-off process when the crystal has a certain size or more. In particular, in the case of increasing the area of the crystal as in the solar cell, it becomes difficult to practically employ the above method.
Also, Japanese Patent Application Laid-Open No. 6-45622 discloses that after a porous silicon layer is formed in the surface region of a silicon wafer surface by anodization, the porous layer is peeled off from the wafer, and after the peeled porous layer is fixed onto a metal substrate, an epitaxial layer is formed on the porous layer, to thereby fabricate a thin-film crystal solar cell. However, in this method, since the metal substrate is exposed to a high-temperature process, impurities are liable to be mixed into the epitaxial layer, which leads to a problem that the characteristic is restricted. Also, as realized in the amorphous silicon solar battery, if a thin semiconductor layer is formed on a flexible substrate, for example, a film of polymer such as polyimide, it can be located on a substance having a curved surface, thus expecting the enlargement of an applied field. However, because a high temperature is required in a process of the above-mentioned single-crystal or polycrystal silicon solar cell, it is difficult to use a substrate having poor heat resistance at a high temperature.
By the way, Japanese Patent Application Laid-Open No. 8-213645 discloses that an active layer of the solar cell is allowed to epitaxially grow on amorphous silicon formed in the surface region of a silicon wafer by anodization, and thereafter the active layer can be peeled off from a portion of the porous silicon layer. Therefore, not only the expensive single-crystal substrate can be repeatedly utilized, but also a high-efficiency solar cell can be formed on a flexible low-heat-resistant substrate. However, according to the disclosure of Japanese Patent Application Laid-Open No. 8-213645, the epitaxial growth of the active layer is conducted by the CVD method. In the CVD method, a source gas such as dichlorosilane (SiH.sub.2 Cl.sub.2) or trichlorosilane (SiHCl.sub.3) and a large amount of hydrogen gas are used. When a silicon film is deposited to 20 to 50 .mu.m in thickness using a large amount of the expensive gas, it is correspondingly expensive and considerably disadvantageous from the viewpoint of cost in comparison with amorphous silicon which is required to be deposited to at most 0.5 to 1.0 .mu.m in thickness.